1. Field of the Invention
The present invention relates to a Raman amplifier for Raman-amplifying a plurality of signal channels of signal light by utilizing Raman scattering effect, a method of controlling the Raman amplifier, and an optical communication system including the Raman amplifier.
2. Related Background Art
For improving the information transmission capability of optical communication systems, the wavelength division multiplexing transmission of signal light has become widespread. Such an optical communication system utilizes optical amplifiers directly amplifying the signal light during the transmission. Here, it is important for the optical amplifiers to attain amplification over a wide wavelength range (optical frequency range) and flatten the power spectrum of amplified signal light in the wavelength range (optical frequency range). Typical optical amplifiers are optical fiber amplifiers, such as those doped with rare-earth elements and Raman amplifiers.
Erbium-doped optical fiber amplifiers doped with erbium elements, for example, are typical as rare-earth-element-doped optical fiber amplifiers which are one of types of optical fiber amplifiers. In the erbium-doped optical fiber amplifiers, pumping light having a pumping channel in the wavelength band of 980 nm (corresponding to the band of 306.1 THz in terms of the optical frequency) or 1480 nm (corresponding to the optical frequency band of 202.7 THz) is made incident on an optical fiber doped with erbium element, so as to generate a population inversion state in the erbium-doped optical fiber. When signal light in the wavelength band of 1550 nm (corresponding to the optical frequency band of 193.5 THz) is made incident on the erbium-doped optical fiber generating the population inversion state, the signal light is amplified. The amplification band of such a rare-earth-doped optical fiber amplifier is determined by the kind of rare-earth element added to the optical fiber. For example, in the case of erbium-doped optical fiber amplifiers, the wavelength range in which the amplified signal light has a power fluctuation (power spectrum flatness) of 1 dB or less is 1540 nm (194.8 THz) to 1560 nm (192.3 THz). Further, equalizing filters for correcting the flatness of power spectrum and the like have been employed, thus making it possible for the wavelength range to become 1530 nm (196.1 THz) to 1560 nm (192.3 THz).
However, in response to urgent demands for improving the information transmitting capability of optical communication systems in recent years, the high-density multiplexing of signal light aimed at improving the capability has been reaching its limit. Therefore, in order to further improve the information transmitting capability of optical communication systems, it has been becoming necessary to expand the amplification wavelength band (optical frequency band) in which the amplified signal light has a flat power spectrum.
Raman amplifiers, which are the other type of optical fiber amplifiers, supplies a plurality of pumping channels of pumping light having their respective center wavelengths in the wavelength band of 1400 nm (corresponding to the optical frequency band of 214.3 THz), for example, to an optical fiber constituting at least a part of a transmission line through which the signal light propagates and Raman-amplifying the signal light, so as to utilize Raman scattering effect yielding an amplification peak in a wavelength band (optical frequency band) shifted by 100 nm on the longer wavelength side (by 13 THz on the lower optical frequency side) from the wavelength of pumping light (optical frequency).
Raman amplifiers can amplify signal light in the wavelength band of 1500 nm (corresponding to the optical frequency band of 200.0 THz) when a pumping light source in the wavelength band of 1400 nm (corresponding to the optical frequency band of 214.3 THz), which is typically used in erbium-doped optical fiber amplifiers, is utilized as their pumping light source, for example. When a plurality of pumping channels of pumping light having wavelengths (optical frequencies) different from each other are employed as appropriate, the Raman amplifiers can greatly expand the wavelength band in which the Raman-amplified signal light attains a flat power spectrum as compared with that in erbium-doped optical fiber amplifiers. Japanese Patent Application Laid-Open No. 2000-98433 discloses one comprising a pumping light generating section for generating a plurality of channels of pumping light having different wavelengths (optical frequencies), a WDM coupler for multiplexing the individual pumping channels of pumping light, and an optical output power controller for detecting the output light (Raman-amplified signal light) from the optical amplifier and controlling the pumping light generator according to the result of detection. For reducing the wavelength dependency of amplification to such an extent that no amplification output equalizing filter is necessary, the center wavelength interval in pumping light is 6 nm or more but 35 nm or less.